Our long-term goal is to determine the energetic-structure-function relationship in apolipoproteins and lipoproteins to gain a better insight into the molecular mechanisms of lipoprotein action in normal and in atherosclerotic states. The emphasis is on the folding, structure and stability of apolipoproteins in solution and on lipoproteins, particularly on model discoidal HDL and human plasma or reconstituted LDL. The balance between HDL, LDL and their subclasses in plasma determines the probability of developing cardiovascular disease and stroke. Structural stability of lipoproteins is essential for their functions, yet the precise mechanism of their stabilization is unknown. Furthermore, HDL and LDL are extensively re-modeled by plasma factors during metabolism; for example, LDL fusion in the arterial wall is a key event in early atherosclerosis. However, the molecular mechanisms of HDL and LDL remodeling and fusion are not wellunderstood. Our research is aimed at detailed understanding of the energetic and structural basis underlying stability and re-modeling of HDL and LDL. Our preliminary studies of reconstituted discoidal HDL have revealed a novel kinetic mechanism of lipoprotein stabilization; they showed that lipoprotein destabilization and protein dissociation lead to particle fusion that involves high energy barriers. We demonstrated that a similar fusion-based kinetic mechanism confers stability to plasma HDL and LDL, and thus provides a universal natural strategy for lipoprotein stabilization. In the proposed work we will obtain detailed molecular determinants for the kinetic stability of discoidal HDL. The focus will be on the role of protein size, hydrophobicity, charge residue distribution, and lipid composition in the disks stability, with an emphasis on apoA-1-based proteins and peptides. Discoidal HDL of controlled composition will be reconstituted and analyzed by using an integrated spectroscopic, calorimetric, and electron microscopic approach. The kinetics of protein-lipid association, which will be analyzed by absorption and circular dichroism spectroscopy, will help to determine the role of the protein primary and secondary structure on the lipid binding pathway. Molecular determinants for LDL stability and fusion will be obtained, with an emphasis on the effects of protein and lipid oxidation, lipid composition, and apoB C-terminal truncations on the structure and stability of plasma or reconstituted lipoproteins. The energetic and structural analysis of HDL and LDL, such as this proposal, may lead to identification of compounds that promote or inhibit pro-atherogenic lipoprotein transformations such as fusion, and thereby help to develop new therapies for coronary artery disease and other lipoprotein-related disorders. Comparative studies of antifusogenic effects of apolipoproteins and peptides may also help to design apolipoprotein-based peptides with optimized antiviral properties, while identification of key determinants for LDL stability may help to design LDL-based drug carriers.